2 Answers
2

All the source files are located at /etc/apt/sources.list.d/. You will need root privileges to edit the ppa files. See if you added ppa are there as a file. If they are located there see if the content of files are commented or not. If they are not in the above location you have to manually add the ppas using sudo add-apt-repository command.

This I assume is after a distribution update? if so this is the default action but they will be still in the "Other Software" tab but marked as disabled. You get to this by the edit menu in the Ubuntu Software Centre and select software sources. And check the PPAs you want to enable.

Reload the sources. Ubuntu automatically updates the version from ie. "maverick" to "natty". If a PPA has not been updated for the new version, you will receive errors when you reload the sources. For each error go back and change "natty" back to "maverick" (or whichever version you updated from). This is usually safe as the packages generally work form version to version. Reload sources again and the PPA's should re-appear. You can wait a few weeks and try again with the newer version in case the PPA maintainer has updated his PPA.